1. Field of the Invention
The present invention relates to a content retrieval device, and more particularly, to a content retrieval device which is constructed to permit a plurality of connection methods to be used and which retrieves content data from a server via a communication network under an optimum connection method.
2. Description of the Background Art
In recent years, browsing of home pages (Web pages) and exchanges of emails on the Internet have attained great popularity. To access the Internet, the user operates a content retrieval device typified by a cellular phone. The content retrieval device first establishes a connection to a user-subscribing network (for example, a mobile communication network) for access to the Internet. The content retrieval device then accesses a server on the Internet and retrieves content data which is typified by a home page or an email via the network according to the operation by the user.
Conventionally, partly because of a low transfer rate in a network, servers mainly handled small-size content data such as text files and still picture files. However, with recent technological advances, the content retrieval devices have achieved enhanced performance, and the transfer rate in a network has dramatically improved. Accordingly, servers are now able to handle large-size content data such as moving picture files and audio files.
Conventional content retrieval devices access a network under either one of two connection methods; a circuit switching connection or a packet switching connection. In the circuit switching connection, one physical communication path is established between one caller and one call receiver. Since the caller and the call receiver occupy one communication path from the start of a call to the finish thereof in the circuit switching connection, data communication between the caller and the call receiver is free from influence of other data communications. That is, the communication delay, which means the time which is required to deliver data from a sender to a receiver, can be made substantially constant, and thus it is easy to assure a transfer rate. With this feature, the circuit switching connection is suitable for occasions of transmitting large-size content data to the same receiver, such as multimedia phones and moving picture distribution.
In the packet switching connection, a communication path is not occupied by one call but is shared with other calls, which is contrary to the circuit switching connection. On the shared communication path, data is divided into packets and is transmitted together with other packets for other call exchanges. The packet switching connection therefore permits an effective use of communication path resources which resultantly provides a reduction in the communication cost. However, the packet switching connection experiences and/or causes problems such as the loss of packets and the reversal of the order of arrival of the packets, and thus, the packet switching connection fails to achieve a constant communication delay in contrast to the circuit switching connection. That is, in the packet exchanging connection, it is not easy to assure a transfer rate. Moreover, since packets for one call exchange must be distinguished from the packets for other call exchanges, each packet includes identifiers representing the sender and the receiver in addition to the data to be transmitted. The effective transfer rate is therefore lower in the packet switching connection than in the circuit switching connection. The effective transfer rate as used herein refers to the transfer rate for data only, excluding control information such as identifiers. In view of the above, the packet switching connection is suitable for occasions where a communication delay does not cause a significant problem or where data communication is not always active throughout the call period, such as for the exchange of emails.
Conventionally, the content retrieval devices used only either one of the circuit switching connection and the packet switching connection. Recently, there has been developed a content retrieval device that can selectively use either the circuit switching connection or the packet switching connection. An example of such a content retrieval device is an inter-LAN connection device disclosed in Japanese Patent Gazette No. 2625388. The inter-LAN connection device is applied to systems which execute data communication via ISDN (Integrated Services Digital Network). The inter-LAN connection device monitors the data transfer amount on a communication path, and selects either the circuit switching connection or the packet switching connection based on the data transfer amount and the communication traffic amount, which means the communication density of data on a communication path, for each transaction.
The inter-LAN connection device monitors data communication actually executed and selects either the circuit switching connection or the packet switching connection based on the status of the monitored data communication. Therefore, it is difficult for the inter-LAN device to select the particular connection method which is suitable for incoming or arriving data communication. The inter-LAN connection device has another problem as follows. The connection method may be switched depending on the status of the data communication. In such an event, a communication delay is caused by the time which is required to complete the switching from one connection method to the other when a continuous data communication without interruption is required such as when a moving picture file is transmitted, for example. In view of the above, the inter-LAN connection device is not suitable for data having a nature such that a communication delay and interruption of data communication are fatal.
To solve the above-described problems, the inter-LAN connection device is provided with a transaction information setting section that sets an attribute of the data which is exchanged for each transaction as transaction information. By referring to the transaction information setting section, incoming data to be exchanged is predicted and a suitable connection method for the data is selected. However, on the Internet, various types of data such as text files, moving picture files, and audio files are available. Therefore, it is difficult for the inter-LAN connection device to correctly predict incoming data to be exchanged.